Anencephaly represents a defective closure of the anterior portion of the neural tube, what leads to a lethal prognosis, and is characterized by the absence of the cranial vault alongside the exposure of neural tissue. On average, it represents one third of all CNS malformations diagnosed by ultrasound. Its global incidence is of around 1 out of 1.000 births (Isfer, 1996).

The cranial development, which is completed around the 10th week, does not occur. So, the exposed cerebral tissue gradually undergoes a degeneration to the contact with the amniotic fluid. Whenever the diagnosis is carried out between the 10th and 14th week, the brain can still be examined, displaying irregular contours. However, after the 17th week, the destruction of the cerebral tissue is completed, there being the classic aspect of the anencephalic fetus. It is usual its association with facial anomalies such as cleft, as well as with anomalies of the cervical vertebrae (bifid spine) and omphalocele. The echographic diagnosis can be given in around 12 weeks of gestation.